1. Field of the Invention
The present invention relates to a flip-chip type nitride semiconductor light emitting diode, and more particularly, to a flip-chip type nitride semiconductor light emitting diode, which has an n-side electrode comprising at least one finger diagonally formed on an n-type nitride semiconductor layer of a nitride semiconductor light emitting diode having a substantially rectangular cross section, thereby lowering a driving voltage while suppressing heat generation.
2. Description of the Related Art
Generally, a nitride semiconductor refers to a III-V group semiconductor crystal, such as GaN, InN, AlN and the like, and is utilized for a light emitting device, which can emit light in a short wavelength range (from ultraviolet light to green light), especially blue light.
Since the nitride semiconductor light emitting diode is manufactured by use of an insulating substrate, such as a sapphire substrate or a SiC substrate, which satisfies lattice matching requirements for crystal growth, it has a planar structure in which two electrodes respectively connected to p-type and n-type nitride semiconductor layers are aligned substantially horizontally on an upper surface of the light emitting structure. Due to such a structural characteristic, the nitride semiconductor light emitting diode has been actively developed for a flip-chip type light emitting device.
In FIG. 1a, one example of a flip-chip type light emitting device having a conventional nitride semiconductor light emitting diode mounted thereon is illustrated. In FIG. 1a, the conventional nitride semiconductor light emitting diode 10 of the flip-chip type light emitting device is a small size diode having a cross sectional area of 400×400 μm2 or less, and is driven at a low electric current of 20 mA or less.
The conventional nitride semiconductor light emitting diode 10 comprises an n-type nitride semiconductor layer 12, an active layer 13, a p-type nitride semiconductor layer 14, and an ohmic contact layer 15 sequentially formed from the bottom on a sapphire substrate 11. The nitride semiconductor light emitting diode 10 further comprises an n-side electrode 16 formed on an exposed region of an upper surface of the n-type nitride semiconductor layer 12, and a p-side electrode 17 formed on an upper surface of the ohmic contact layer 15.
The flip-chip type light emitting device 20 having such a conventional nitride semiconductor light emitting diode 10 mounted thereon has the construction in which the nitride semiconductor light emitting diode 10 is mounted on a supporting substrate 21 by fusing the respective electrodes 16 and 17 onto respective lead patterns 22a and 22b through conductive bumps 24a and 24b. In such a flip-chip type light emitting device 20, since the sapphire substrate 11 of the light emitting diode 10 is transparent, it can be used as a light emitting surface. The ohmic contact layer 15 is required to have a higher reflectivity which can reflect light emitted from the active layer 13 towards the light emitting surface (that is, the sapphire substrate 11) while providing an ohmic contact with the p-type nitride semiconductor layer 14.
Meanwhile, referring to FIG. 1b which is a top view of the conventional nitride light emitting diode, the conventional nitride light emitting diode has a substantially rectangular shape, and has the n-side electrode 16 formed adjacent to one corner of the rectangular shape. The conventional nitride semiconductor light emitting diode with the electrode structure as described above suffers a current crowding phenomenon wherein current is not uniformly spreaded as distance from the n-side electrode 16 is far, and is concentrated on a portion adjacent to the n-side electrode 16.
That is, referring to FIG. 1a, the conventional nitride semiconductor light emitting diode 10 has the planar electrode structure, and in particular, since the p-side ohmic contact layer 15 has a lower specific resistance in comparison to the p-type nitride semiconductor layer 14, there occurs the current crowding phenomenon where a significant amount of electric current is concentrated on a portion A adjacent to the n-side electrode 16 along the ohmic contact layer 15 as indicated by an arrow in FIG. 1a. 
The current crowding phenomenon both increases forward voltage of light emitting diode and lowers brightness of the light emitting diode by reducing light emitting efficiency of a portion of the active layer relatively far from the n-side electrode. Moreover, heat is increasingly generated at the portion A where the electric current is concentrated, thereby significantly reducing reliability of the light emitting diode.
As such, since the conventional nitride semiconductor light emitting diode has the n-side electrode 16 located adjacent to the one corner of the upper surface of the nitride semiconductor light emitting diode, the current density is non-uniformly distributed over the entirety of the light emitting surface, thus the effective area of emitting light is limited and light emitting efficiency per area is low in comparison to a light emitting diode having a vertical structure in which two electrodes are located on upper and lower surfaces of the light emitting structure, respectively. Moreover, in comparison to the light emitting diode of the vertical structure, since electric current between the two electrodes is concentrated on a path having a shortest distance, the conventional nitride semiconductor light emitting diode has a narrower current path on which current density is concentrated, and since the electric current flows in the horizontal direction, it has a higher driving voltage due to a large series resistance.
In particular, due to these problems as described above, there is a significant difficulty in ensuring high power output from the light emitting diode for an illumination device having a large size (for example, 1,000×1,000 μm2).